docs: def on_touch_move(self, touch):
          """If an entity is selected, drag it."""
          if hasattr(self, '_lasttouch') and self._lasttouch == touch:
              return
          if self.app.selection in self.selection_candidates:
              self.selection_candidates.remove(self.app.selection)
          if self.app.selection:
              if not self.selection_candidates:
                  self.keep_selection = True
              ret = super().on_touch_move(touch)
              return ret
          elif self.selection_candidates:
              for cand in self.selection_candidates:
                  if cand.collide_point(*touch.pos):
                      self.app.selection = cand
                      cand.selected = True
                      touch.grab(cand)
                      ret = super().on_touch_move(touch)
                      return ret